The present invention relates to a sound image localization control device for processing sound image localizing signals. The localization of a sound image provided by a 2-channel speaker or a 2-channel headphones is to localize the sound image as if it were located in a position other than a position of the speaker or the headphone. In order to realize such sound image localization, digital filters are used which are constructed such that sound pressure around eardrums of a listener which is caused by a virtual sound source at a desired location becomes equal to a sound pressure caused by the speakers or the headphone by allowing crosstalk in a head related transfer function (HRTF) measured at a head of a listener or a dummy head in a case of the 2-channel speaker or by partially cancelling a headphone characteristics or providing crosstalk in the headphone characteristics in a case of the headphone.
FIG. 1 shows a principle of a sound image localization control device proposed by the same assignee of this application and disclosed in Japanese Patent Application Laid-open No. H6-17839.
According to the device shown in FIG. 1, transfer function cfLx and cfRx at a desired location x are preliminarily provided as coefficients for realizing the device by convolver processing in such as Finite Impulse Response filters (FIR filters) or Infinite Impulse Response Filters (IIR filters) and, in a case where a sound source X is to be located at a desired position, the transfer function cfLx based on an actual measurement and stored in an ROM is transferred to the FIR digital filter to perform a convolution progressing of signals from the sound source X and to reproduce the thus processed signals by a pair of speakers SP1 and SP2.
Data preliminarily stored in the ROM are obtained through a measuring system shown in FIG. 2.
According to the system shown in FIG. 2, a pair of microphones ML and MR are set on ears of a dummy head (or human head) DM. Sound from a speaker SP which includes source sounds (reference data) refL and refR and sounds to be measured (measurement data) L, and R is received by the microphones ML and MR and the reference data refL and refR and the measurement data L and R are recorded in recorder DATs in synchronism with each other. The transfer function which is wave-shaped in a predetermined manner on the basis of the recorded data is thus obtained.
In case where the sound source localization is performed by the above mentioned convolver processing, such problems as better feeling of distance with longer impulse response time of the processing system, inversion of forward and rearward sound images, rise of localized sound image (imaginary sound image) in which a listener hears sound from a high level position and localization of a front median sound image in a listener's head in which a sound image located in front of the listener is located in his head may hardly occur.
The simplest method for realizing this localization processing utilizing the convolver processing is to prepare a FIR filter having long convolution coefficient length and to convolute a long filter coefficient determined on the basis of HRTF measured in an echo room by using the above mentioned system.
Since, however, the size of hardware is usually limited, it is impossible to make the impulse response time arbitrarily long. In general, in order to solve this problem, an echo sound structure in the echo room is simulated and a resultant echo sound is added to the sound.
FIG. 3(B) shows a filter construction which is considered generally in lieu of the FIR filter. The filter shown in FIG. 3(B) is constructed with delay elements (D0 to D6) and IIR filters. The impulse response waveform in this construction is shown in FIG. 3(C).
On the other hand, FIG. 3(A) shows the impulse response waveform when the filter is constructed with using FIR filters having long convolution coefficients. The impulse response waveform shown in FIG. 3(A) is similar to a desired impulse response waveform. As is clear from these waveforms, when the filter is constructed with the IIR filters, the reproducibility of response waveform similar to the desired impulse response waveform is low. That is, although the filter constructed with IIR filters is advantageous in that it can be realized with simplified construction simplified by an extent corresponding to in the order of a single digital signal processor IC chip, it is defective in that a listener hears sounds as if a sound image were located within his head or in the vicinity of a surface of the head or in an elevated level, so that the distance feeling to a sound image is lost.